ARC REF 150 Buzzing noise

I have an ARC REF 150 amp with over 4000 hours on it, and recently it has developed a buzzing noise.  This appears after about 30 minutes of use, and seems to get steadily louder.  I thought it might be a tube.   I had KT150 power tubes in it and I swapped those out  for KT120's and re-biased.  No change.  I then swapped out the 6H30 input tubes and re-biased again.  Buzzing is still there.  It is very difficult to locate the sound on the boards.    I have also had several blown fuses, this has been in the midst of a move and may or may not be reIated to the buzzing.  I have to assume this needs a repair, but I am wondering if anyone has a guess or even better, experience with the problem.  I am in the Denver area and if anyone knows of a local repair opportunity, I would appreciate knowing that too.  I do not look forward to the expense and delay of sending it back to ARC repair.  Thanks.  Scott
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Is it possible this is from the transformer?

If so, it's often caused by DC on the AC line. This is often caused by switching power supplies, including lights, PC's, etc. that only take current from one direction.

Turn off everything else in the home from the panel. See if that fixes it.

If you can't disable the root cause, a balanced AC conditioner is your solution.
Agree that this sounds like transformer hum. Hopefully from the external sources as this would be fatally expensive to replace any of the 3 xformers in your amp. Try to eliminate other sources as mentioned above.
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Should have mentioned that the amplifier is plugged into a Bryston BIT-20 power unit, an isolation transformer.  I am in a new house, but only for a few weeks.  I am unsure if I heard the buzzing in the old place.  Perhaps so.  I will try plugging the amp directly into the wall and see if that fixes the problem.  I would have to believe that a failure of one of the three transformers would be fairly unusual?  Thanks for the replies, I will update the post as soon as I have more results.
The speakers are 89 dB.  Up to a few months ago, I didn't hear buzzing at all.  I did remove the amp from the Bryston BIT-20 and the buzzing didn't go away.  I will try turning off other circuits in house next.

It takes an hour or so for the buzzing to appear, as if it is usage related or heat related.
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Moved the power cord to another circuit, quite close to the power panel.  The amp was quiet for about an hour, which got my hopes up, but then started the nasty buzzing again.  It MIGHT be one of the three transformers, but the exact source is hard to locate.  It is obnoxious enough to be a problem, for sure.  Some of you have mentioned DC content in the power, but wouldn't the Bryston BP-20, which is a large isolation transformer (I think) eliminate that if it were present?  BTW, my most recent tests have not had the BP-20 in the power feed.  The Bryston does not seem to be the problem, as the amp acts the same with or without it.

I am going to call ARC service today, will report back on what I hear from them.
Update after last post, there is no time dependency.  No matter when I turn on the unit, it will start to buzz after about an hour.  Anyone know what those transformers cost?
An educated guess but I think you may have damaged the transformer by using KT150 tubes.if this t
is the non SE REF 150 then according to Audio Research to upgrade to SE and use KT150’s some major upgrades needed to be done,including replacing the transformers.

i would put the KT120 tubes back in it and send it to Audio Research for evaluation, inspection and repair.  Also,find out how much to repair and how much to upgrade to SE.  it might be the same price if you have to change the same transformer
Called Evan at Audio Research, seemed very knowledgeable.  He thought the problem was a transformer, the good news is there is a service center near me that can repair it.  I think my cost will be about $ 800 including labor if it turns out to be a transformer.  Given the great sound that this amp makes, I don't mind too much.  I asked him what the official stance from ARC on the use of KT-150's in this amp, his answer was "highly recommended, it makes the amp sound better and the tubes last longer."  So there you are.  I will post updates as this moves along.

I’ll bet minorl is right. The KT150 is much more demanding of transformers than is the KT120. Verify that the tech is replacing the one in your amp with one rated for the KT150, otherwise you may end up right back where you are. Maybe consider getting the complete SE upgrade?
Update on the sad story of my Reference 150.  Took it to Audio Tube Supplies in Denver, they determined that one of the input tubes was out of spec, causing one of the KT150 tubes to become over-biased and over heat.  This in turn (educated guess) caused one of the rubber washers under the transformer to shrink, and the transformer began to hum -- after it gets warm.  In summary, the bad input tube caused the amp to get too hot. The shop placed a sliver of heat resistant neoprene under the washer, and the problem went away.

Lesson learned, check your tube bias regularly and often!  Especially if your tubes have some hours on them. 

It now has a new set of tubes.  I felt cautious and went back to KT120's, although the shop verified that there should not be a problem with using the larger KT150 tubes.  I don't think there is any permanent harm, the amp sounds as good as ever with the new tube set.  But, I am going to send back to ARC for complete check.  It has over 5000 hours.

@scottwsmith I'm having the very same problem right now. Buzz in about 30 minutes, sound gets leaner as the buzz is getting stronger. 

Good to know where to focus. 

KT150's should be alright, I checked with ARC some time ago but I'm still using KT120's as they still have lots of time left. 

KT150 are just fine to use in the Ref150. In fact they should sound much better than the 120s.